In bonding wafers to each other, for example, eutectic bonding is carried out. The bonded surfaces of metal layers of the wafers are oxidized in the atmosphere, thereby forming oxide films at the bonded surfaces. The oxide films are known to interfere with bonding between the wafers. To avoid interference with bonding, in Patent Document 1 and Patent Document 2, the bonded surface of a metal layer of a first wafer and the bonded surface of a metal layer of a second wafer are both subjected to a pre-treatment for removing or reducing oxide films with dilute hydrofluoric acid or a forming gas, or by sputtering.
On the other hand, in Non-Patent Document 1, neither the bonded surface of a metal layer of a first wafer nor the bonded surface of a metal layer of a second wafer is subjected to the pre-treatment for removing or reducing an oxide film. Instead, in Non-Patent Document 1, a relatively large load acts on a pair of wafers stacked on each other so as to apply a mechanical shock to the bonded surfaces of both the metal layers, and thus mechanically break the oxide films, thereby removing the oxide films, and at the same time, achieving eutectic bonding of the bonded surfaces to each other.    Patent Document 1: U.S. Pat. No. 7,442,570    Patent Document 2: Japanese Patent Application Laid-Open No. 2014-107393    Non-Patent Document 1: EV Group (EVG), “Wafer Bonding Processes for the Manufacture of MEMS devices for the Mobile Applications”, SEMICON Taiwan 2012, Sep. 5 to 7, 2012